Connection modification method applicable to user equipment and base station

ABSTRACT

The disclosure is directed to a connection modification method in a dual connectivity scenario. In one of the exemplary embodiments, a UE configures a first MCG bearer for communication with a first base station; configures a SCG bearer for communication with a second base station; generates and transmits a first encrypted PDCP SDU by encrypting a first PDCP SDU of the first MCG bearer by using a first security key, a first TX_HFN and a first PDCN SN associated with the first PDCP SDU; generates and transmits a second encrypted PDCP SDU; receives a third bearer configuration re-configuring the SCG bearer as either a second MCG bearer or a split bearer; generates a third encrypted PDCP SDU by encrypting a third PDCP SDU of the SCG bearer and transmits the third encrypted PDCP SDU after the SCG bearer is reconfigured as the second MCG bearer or the split bearer.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of U.S.A. provisionalapplication Ser. No. 61/952,894, filed on Mar. 14, 2014, U.S.A.provisional application Ser. No. 61/954,578, filed on Mar. 17, 2014, andU.S.A. provisional application Ser. No. 61/990,724, filed on May 9,2014. The entirety of each of the above-mentioned patent applications ishereby incorporated by reference herein and made a part ofspecification.

TECHNICAL FIELD

The disclosure is directed to a connection modification methodapplicable to a user equipment (UE) connected to multiple base stationsand applicable to a base station which provides service to a UEconnected to multiple base stations.

BACKGROUND

A long-term evolution (LTE) system that supports the 3^(rd) GenerationPartnership Project (3GPP) Release 8 standard and/or the 3GPP Release 9standard has been developed by the 3GPP has a successor of a universalmobile telecommunications systems (UMTS) to enhance further performancesof the UMTS so as to satisfy the increasing needs of users. Such LTEsystem would include a new radio interface and a new radio networkarchitecture that provides high data rate, low latency, optimized datapacket, improved system capacity, and improved system coverage. In a LTEsystem, a radio access network (RAN) known as an evolved universalterrestrial radio network (E-UTRAN) may include at least one evolvedNode B (eNB) that would communicate with at least one multiple userequipment (UE). An eNB may also communicate with a core network througha mobility management entity (MME) to reach various network entitiessuch as various gateways and servers within the non-access stratum (NAS)level of the core network.

A LTE-advanced (LTE-A) system, as its name implies, is a more advancedLTE system. A LTE-A communication system would provide, relative to itspredecessors, faster switching between power states, better performanceat the edge of the coverage of an eNB, and also advanced techniques suchas carrier aggregation (CA), coordinated multipoint (CoMP)transmission/reception, UL multiple-input multiple-output (MIMO), and soforth. In order for a UE and an eNB to communicate with each other inthe LTE-A system, the UE and the eNB must meet the standards developedfor a LTE-A system such as the 3GPP Release 10 standard or laterversions.

A small cell controlled by a low-power base station such as a micro cellbase station, pico cell base station, femto cell base station, and soforth, could be deployed to satisfy increasing network traffic resultedfrom increasing demands of mobile electronic devices. A small cell couldbe deployed as hot spots for both indoor and outdoor scenarios. A lowpower base station could generally be defined as a base station havingan operating power lower than that of a macro cell base station such asa macro NodeB, a macro eNB, or other variants. A UE may thussimultaneously communicate with a macro cell base station and a smallcell base station by applying dual connectivity. In this instance, a UEmay transmit or receive user data and control information via both themacro cell base station and the lower power base station. The dualconnectivity may provide for the UE the benefit of an increased datathroughput resulted from the simultaneous dual transmissions with amacro base station.

Any base station may serve two different roles as either a master eNB(MeNB) or a secondary eNB (SeNB), although typically a macro cell basestation would be the MeNB and the small cell base station would be theSeNB. FIG. 1 illustrates a dual connectivity scenario applicable to awireless communication system in which a UE is dual connected to twobase stations. The wireless communication system could be a LTE or LTE-Acommunication system and would include at least but not limited to aMeNB 101, a SeNB 102, a UE 103. The MeNB 101 could be a macro cell basestation and would provide a first coverage area 105, and the SeNB 102could be a small cell base station and would provide a second coveragearea 104 which is smaller than the first coverage area 105. The secondcoverage 104 could either overlap completely with the first coveragearea 105 or could be partially outside of the first coverage area 105.For the wireless communication system of FIG. 1, the UE 103 would beable to communicate with both the MeNB 101 and the SeNB 102 at the sametime. This means that, at some point in time, the UE 103 would becapable of performing an uplink transmission to both a cell of the MeNB101 and a cell of the SeNB 102 as well as a downlink reception from boththe cell of the MeNB 101 and the cell of the SeNB 102.

It should be noted that MeNB 101 and SeNB 102 may operate underdifferent carrier frequencies. Hypothetically, assuming that a carrieraggregation scheme is used by the wireless communication system 100,which is for example a LTE-A communication system, MeNB 101 may operatea first component carrier (CC1) and SeNB may operate under a secondcomponent carrier (CC2), and the frequency spectrum of CC1 does notoverlap with the frequency spectrum of CC2.

FIG. 2A illustrates a general RAN protocol architecture implemented bythe exemplary wireless communication system of FIG. 1. The radioprotocol architecture that a particular radio bearer uses depends on howthe radio bearer is setup. A radio bearer can be a data radio bearer(DRB) for user plane data transmission and/or reception for a signalingradio bearer (SRB) for control plane data transmission and/or reception.A DRB for control plane includes DRB identity, EPS bearer identity,Packet Data Convergence Protocol (PDCP) configuration (e.g. headercompression configuration), logical channel identity and logical channelconfiguration (e.g. priority and logical channel group). A SRBconfiguration includes SRB identity, Radio Link Control (RLC)configuration and logic channel configuration.

In dual connectivity, as shown in FIG. 2B, there could be at least threetypes of dual connectivity radio bearers: Master cell group (MCG) bearer211, secondary cell group (SCG) 212 bearer and split bearer 213. The MCGbearer 211 utilizes radio protocols only located in MeNB (e.g. 101) touse MeNB resources only. SCG bearer 212 utilizes radio protocols onlylocated in the SeNB (e.g. 102) to use SeNB resources. Split bearer 213utilizes radio protocols located in both the MeNB (e.g. 101) and theSeNB (e.g. 102) to use both MeNB and SeNB resources. In general, a DRBcan be a MCG bearer, a SCG bearer, or a split bearer. Also in general, aSRB can be a MCG bearer, a SCG bearer, or a split bearer.

A RB (i.e. DRB or SRB) is associated with one PDCP entity. The PDCPentity is associated with one or two (one for each direction) RLCentities depending on RB characteristic (i.e. uni-directional orbi-directional) and RLC mode (e.g. acknowledged mode or unacknowledgedmode). For a split bearer, a PDCP entity is associated with two AM RLCentities. For encryption/decryption and integrity protection/check, aCOUNT value associated to a PDCP service data unit (SDU) would bemaintained by the UE side as well as base station(s) that provideservices to the UE. The COUNT value is composed of a hyper frame number(HFN) and a PDCP sequence number. A state variable Next_PDCP_TX_SNindicates the PDCP sequence number of the next PDCP SDU to betransmitted by a PDCP entity. A state variable TX_HFN indicates HFNvalue for the generation of COUNT value used for encrypting or integrityprotecting PDCP PDUs to be transmitted. A state variable Next_PDCP_RX_SNindicates the next expected PDCP sequence number by a receiving PDCPentity. A state variable RX_HFN indicates the HFN value for thegeneration of the COUNT value used for the received PDCP PDUs.Mechanisms involving transmissions and receptions of PDCP PDUs with thestate variables as described above could be seen in 3GPP TS 36.323 whichis incorporated by reference.

In dual connectivity, even though a DRB could be initiated as a MCGbearer, a SCG bearer, or a split bearer, the DRB could be reconfiguredfrom one dual connectivity radio bearer into another dual connectivityradio bearer. In other words, after a MeNB sets up a DRB, the MeNB mayreconfigure a DRB from a first dual connectivity radio bearer into asecond dual connectivity radio bearer. Various possibilities aredescribed below.

A first possibility involves a first SCG bearer that is changed into asecond SCG bearer. That would occur in response to a DRB that has beenconfigured as a first SCG bearer under a first SeNB but is reconfiguredto be a second SCG bearer under a second SeNB because of a SeNB change.

A second possibility involves a SCG bearer that is changed into a MCGbearer. That would occur in response to a DRB that has been configuredas a SCG bearer under a SeNB but is reconfigured to be a MCG bearerunder a MeNB because of a circumstance such as the removal of the SeNB.

A third possibility involves a SCG bearer that is changed into a splitbearer. That would occur in response to a DRB that has been configuredas a SCG bearer under a SeNB but is reconfigured to be a split bearerunder MeNB because of a circumstance such as offloading of traffic tothe SeNB.

A fourth possibility involves a MCG bearer that is changed into a SCGbearer. That would occur in response to a DRB that has been configuredas a MCG bearer under a MeNB but is reconfigured to be a SCG bearerunder a SeNB because of a circumstance such as offloading traffic to theSeNB.

A fifth possibility involves a split bearer that is changed into a SCGbearer. That would occur in response to a DRB that has been configuredas a split bearer but is reconfigured to be a SCG bearer because of acircumstance such as offloading traffic to the SeNB.

A sixth possibility involves a split bearer that is changed into a MCGbearer. That would occur in response to a DRB that has been configuredas a split bearer but is reconfigured to be a MCG bearer because of acircumstance such as the removal of the SeNB.

A seventh possibility involves a first split bearer that is changed intoa second split bearer. That would occur in response to a DRB that hasbeen configured as a first split bearer but is reconfigured to be a MCGbearer because of a SeNB change.

An eighth possibility involves a MCG bearer that is changed into a splitbearer. That would occur in response to a DRB that has been configuredas a MCG bearer under MeNB but is reconfigured to be a split bearerbecause of a circumstance such as offloading traffic to a SeNB.

Mechanisms involving a DRB changing from one dual connectivity radiobearer into another dual connectivity radio bearer could be seen fromreferences such as 3GPP TS 36.331 v12.1.0 and 3GPP R2-141857 which areincorporated by reference.

FIG. 3 illustrates a signaling diagram of a secondary cell group(SCG)/SeNB modification procedure. The SCG modification procedure wouldbe initiated by a SeNB and would be used to perform configuration changeof the SCG within the same SeNB. In step S301, a SeNB (e.g. SeNB 102)may transmit a SCG Modification Request message via the X2 interfaceapplication protocol (AP) in order to request a SCG modification byproviding a new radio resource configuration of the SCG by a RadioResource Control (RRC) container in the SCG Modification Requestmessage. In step S302, in response to the SCG Modification Requestmessage being received and accepted by a MeNB (e.g. MeNB 101), the MeNBmay transmit to a UE (e.g. UE 103) a RRCConnectionReconfigurationmessage which includes the new radio resource configuration of SCGaccording to the SCG Modification Request message. In step S303, the UEmay apply the new radio resource configuration of SCG and subsequentlytransmit a RRCConnectionReconfigurationComplete message back to the MeNBas a reply. Assuming that synchronization between the UE and the SeNB isnot required to execute the new radio resource configuration of SCG theUE may perform UL transmissions after having applied the newconfiguration. If the new radio resource configuration of SCG requiressynchronization between the UE and the SeNB, in step S305 the UE mayinitiate a Random Access (RA) procedure. In step S304. The MeNB mayreply a SCG Modification Response to the SeNB by transmitting aRRCConnectionReconfigurationComplete message via the X2-AP.

Under the circumstance in which the UE is unable to comply with (partof) the radio resource configuration of SCG as included in theRRCConnectionReconfiguration message back in step S302, the UE mayperform a reconfiguration failure procedure. The order of the UEtransmitting the RRCConnectionReconfigurationComplete message andperforming the RA procedure toward the SCG has not been defined. Thesuccess of the RA procedure towards the SCG is not required for asuccessful completion of the RRCConnectionReconfiguration procedure. Theprimary SCell (PSCell) in SCG could be changed with the SCG Modificationprocedure. The SeNB may decide whether the RA procedure is requiredaccording to, for example, whether an old special SCell or a new specialSCell belongs to the same TAG The SeNB may trigger the release of SCGScells(s) other than PSCell, and the MeNB cannot reject.

FIG. 4 illustrates a signaling diagram of a SCG addition/MeNB triggeredmodification procedure. The SCG addition procedure would typically beinitiated by the MeNB to add the first cell of the SCG A MeNB may use asame or similar procedure to initiate an addition or a release of SCGcells and of SCG bearers. For all SCG modifications other than releaseof the entire SCG a SeNB would generate the signaling toward a UE. TheMeNB may request to add particular SCells to the SeNB, and the SeNB mayreject. By using the modification procedure, the MeNB may trigger therelease of SCG SCells(s) other than pSCell, and in this case the SeNBcannot reject.

In step S401, the MeNB would transmit, via the X2-AP, a SCGaddition/modification request message including a MCG configuration and(part of) UE capabilities for UE capability coordination to be used as abasis for the reconfiguration by the SeNB. In case of SCG addition andSCG SCell addition request, the MeNB can provide the latest measurementresults (FFS for which SeNB cells). The SeNB may reject such request. Instep S402, assuming that the SeNB accepts the MeNB request, the SeNBwould initiate a SCG Modification procedure as specified in 10.1.2x.1 inR2-141857 and would provide a subsequent response which may be before orafter the SCG Modification procedure. A SCG change procedure could beused to change a configured SCG of one SeNB to another SeNB in the UEand would be realized by a SCG Modification procedure. ARRCConnectionReconfiguration message transmitted during the SCGmodification procedure would include information for the release of thesource SCG.

In the aforementioned dual connectivity scenario, a UE would, at most ofthe time, connect to both a MeNB and a SeNB. However, as a UE is moving,the E-UTRAN that provides wireless service for the UE may change theMeNB and/or the SeNB for the UE. It should be noted that this scenariodifferent from a typical handover scenario in which the UE is handoverfrom a base station to another. For dual connectivity scenario, the UEwould be simultaneously connected to both MeNB and SeNB and typicallyone of the MeNB and SeNB might change at a time.

However, during such period of changing the MeNB and/or SeNB for the UE,the data transmission and data reception would likely be eithersuspended or failed. The suspension or failure might impose potentiallynumerous difficulties for the network which attempts to implementlossless operations at all times. As an example, the encryption anddecryption process would involve using a TX_HFN as shown in FIG. 2C.However, if a transmission has failed at TX_HFN=2 due to e.g. changingthe SeNB to another eNB for the UE, it is uncertain what TX_HFN valueshould be used after the SeNB is changed to the other eNB.

Therefore, how to resume or recover data transmission and data receptionafter changing the MeNB and/or SeNB could be one of the designconsiderations as the current wireless technology moves forward.

SUMMARY OF THE DISCLOSURE

The disclosure is directed to a connection modification methodapplicable to a user equipment (UE) connected to multiple base stationsand applicable to a base station which provides service to a UEconnected to multiple base stations.

In one of the exemplary embodiments, the disclosure is directed to aconnection modification method applicable to a user equipment (UE) whichis capable of dually connecting to a first base station and a secondbase stations. The method would include not limited to receiving a firstbearer configuration which configures a first master cell group (MCG)bearer for communication with the first base station from the first basestation; configuring a first radio bearer as the first MCG beareraccording to the first bearer configuration; receiving a second bearerconfiguration which configures a secondary cell group (SCG) bearer forcommunication with the second base station from the first base station;configuring a second radio bearer as the SCG bearer according to thesecond bearer configuration; generating a first encrypted Packet DataConvergence Protocol (PDCP) service data unit (SDU) by encrypting afirst PDCP SDU of the first radio bearer by using a first security key,a first transmitter hyper frame number (TX_HFN) and a first PDCNsequence number (SN) associated with the first PDCP SDU; transmittingthe first encrypted PDCP SDU to the first base station; generating asecond encrypted PDCP SDU by encrypting a second PDCP SDU of the secondradio bearer by using a second security key, a second TX_HFN and asecond PDCP SN associated with the second PDCP SDU; transmitting thesecond encrypted PDCP SDU to the second base station; receiving a radioresource control (RRC) message comprising a third bearer configurationre-configuring the SCG bearer to be either a second MCG bearer or asplit bearer from the first base station; re-configuring the secondradio bearer either as the second MCG bearer or the split beareraccording to the third bearer configuration; generating a thirdencrypted PDCP SDU by encrypting a third PDCP SDU of the second radiobearer by using the first security key, the second TX_HFN and a thirdPDCP SN associated with the third PDCP SDU, after the second radiobearer is reconfigured to the second MCG bearer or the split bearer; andtransmitting the third encrypted PDCP SDU to the first base station.

In one of the exemplary embodiments, the disclosure is directed to aconnection modification method applicable to a user equipment (UE)capable of dually connecting to a first and a second base stations. Themethod would include not limited to receiving a first bearerconfiguration configuring a first master cell group (MCG) bearer forcommunication with the first base station from the first base station;configuring a first radio bearer as the first MCG bearer according tothe first bearer configuration; receiving a second bearer configurationconfiguring a second MCG bearer or a split bearer for communication withthe first base station from the first base station; configuring a secondradio bearer as the second MCG bearer or the split bearer according tothe second bearer configuration; receiving a third bearer configurationconfiguring a first secondary cell group (SCG) bearer for communicationwith the second base station, from the first base station; configuring athird radio bearer as the first SCG bearer according to the third bearerconfiguration; generating a first encrypted Packet Data ConvergenceProtocol (PDCP) service data unit (SDU) by encrypting a first PDCP SDUof the first radio bearer by using a first security key, a firsttransmitter hyper frame number (TX_HFN) and a first PDCP sequence number(SN) associated with the first PDCP SDU; transmitting the firstencrypted PDCP SDU to the first base station; generating a secondencrypted PDCP SDU by encrypting a second PDCP SDU of the second radiobearer by using the first security key, a second TX_HFN and a secondPDCP SN associated with the second PDCP SDU; transmitting the secondencrypted PDCP SDU to the first base station; generating a thirdencrypted PDCP SDU by encrypting a third PDCP SDU of the third radiobearer by using a second security key, a third TX_HFN and a third PDCPSN associated with the third PDCP SDU; transmitting the third encryptedPDCP SDU to the second base station; receiving a radio resource control(RRC) message comprising a fourth bearer configuration re-configuringthe second MCG bearer or the split bearer to be a second SCG bearer;re-configuring the second radio bearer as the second SCG beareraccording to the fourth bearer configuration; generating a fourthencrypted PDCP SDU by encrypting a fourth PDCP SDU of the second radiobearer by using the second security key, the second TX_HFN and a fourthPDCP SN associated with the fourth PDCP SDU, after the second radiobearer is reconfigured to the second SCG bearer; and transmitting thefourth encrypted PDCP SDU to the second base station.

In one of the exemplary embodiments, the disclosure is directed to aconnection modification method applicable to a first base stationserving a user equipment (UE) capable of dually connecting to the firstbase station and a second base station. The method would includereceiving a first SCG configuration to establish a first SCG bearer forthe UE from the second base station; transmitting a first radio resourcecontrol (RRC) message comprising the first SCG configuration, to the UE;transmitting a first SCG addition request comprising the first SCGconfiguration for establishing a second SCG bearer for the UE, to athird base station; receiving a first SCG addition response comprising asecond SCG configuration which is based on the first SCG configurationin response to transmitting the first SCG addition request, from thethird base station; and transmitting a SCG release request to releasethe first SCG bearer, to the second base station; transmitting a secondRRC message comprising the second SCG configuration to establish thesecond SCG bearer, to the UE.

In one of the exemplary embodiments, the disclosure is directed to aconnection modification method applicable to a base station. The methodwould include not limited to transmitting a first radio resource control(RRC) message to configure a first master cell group (MCG) bearer forcommunication to a UE; transmitting to the UE a first Packet DataConvergence Protocol (PDCP) service data unit (SDU) encrypted with afirst security key; receiving a first secondary cell group (SCG)configuration configuring a first SCG bearer and a second SCGconfiguration configuring a second SCG bearer, from another basestation; transmitting at least one second RRC message to configure thefirst SCG bearer and the second SCG bear in response to receiving thefirst SCG configuration and the second SCG configuration; transmitting amessage to release the first SCG bear, to the other base station;receiving a transmitter hyper frame number (TX_HFN) and aNext_PDCP_TX_SN of the first SCG bearer, from the other base station;transmitting a third RRC message reconfiguring the first SCG bearer tobe a second MCG bearer or a split bearer to the UE; and transmitting asecond PDCP SDU encrypted by using the first security key, the TX_HFNand the Next_PDCP_TX_SN, via the MCG bearer or the split bearer.

In one of the exemplary embodiments, the disclosure is directed to aconnection modification method applicable to a base station. The methodwould include not limited to: transmitting a first secondary cell group(SCG) configuration configuring a first SCG bearer for a UE to anotherbase station; receiving a message requesting establishing a second SCGbearer replacing a second MCG bearer or a split bearer of the UE, fromthe other base station; receiving a transmitter hyper frame number(TX_HFN) and a Next_PDCP_TX_SN of the second MCG bearer or the splitbearer from the other base station; generating an encrypted PDCP SDU byencrypting a PDCP SDU using a security key, the TX_HFN and theNext_PDCP_TX_SN; and transmitting the encrypted PDCP SDU via the secondSCG bearer to the UE.

In order to make the aforementioned features and advantages of thepresent disclosure comprehensible, exemplary embodiments accompaniedwith figures are described in detail below. It is to be understood thatboth the foregoing general description and the following detaileddescription are exemplary, and are intended to provide furtherexplanation of the disclosure as claimed.

It should be understood, however, that this summary may not contain allof the aspect and embodiments of the present disclosure and is thereforenot meant to be limiting or restrictive in any manner. Also the presentdisclosure would include improvements and modifications which areobvious to one skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the disclosure, and are incorporated in and constitutea part of this specification. The drawings illustrate embodiments of thedisclosure and, together with the description, serve to explain theprinciples of the disclosure.

FIG. 1 illustrates a dual connectivity scenario applicable to a wirelesscommunication system in which a UE is dual connected to two basestations.

FIG. 2A illustrates a general RAN protocol architecture implemented bythe exemplary wireless communication system of FIG. 1.

FIG. 2B illustrates three types of radio bearers and reconfiguration ofradio bearer type in dual connectivity.

FIG. 2C illustrates an encryption procedure as an example of adifficulty that might occur when a transmission has failed or suspended.

FIG. 3 illustrates a signaling diagram of a secondary cell group (SCG)modification procedure.

FIG. 4 illustrates a signaling diagram of a SCG addition/MeNB triggeredModification procedure.

FIG. 5 illustrates the hardware components of a UE in terms offunctional block diagrams in accordance with one of the exemplaryembodiments of the disclosure.

FIG. 6 illustrates the hardware components of a base station in terms offunctional block diagrams in accordance with one of the exemplaryembodiments of the disclosure.

FIG. 7 illustrates a first exemplary embodiment of the disclosure.

FIG. 8 illustrates a second exemplary embodiment of the disclosure.

FIG. 9A illustrates a scenario of altering a SCG bearer as a result of aSeNB change in accordance with the third exemplary embodiment of thedisclosure.

FIG. 9B illustrates a third exemplary embodiment of the disclosure.

FIG. 10 illustrates a fourth exemplary embodiment of the disclosure.

FIG. 11 illustrates a fifth exemplary embodiment of the disclosure.

FIG. 12 illustrates a sixth exemplary embodiment of the disclosure.

FIG. 13 illustrates a connection modification method from theperspective of a user equipment (UE) connected to multiple basestations.

FIG. 14 illustrates a connection modification method from theperspective of a user equipment (UE) connected to multiple basestations.

FIG. 15 illustrates a connection modification method that is from theperspective of a base station which provides service to a user equipment(UE) connected to multiple base stations.

FIG. 16 illustrates a connection modification method that is from theperspective of a base station which provides service to a user equipment(UE) connected to multiple base stations.

FIG. 17 illustrates a connection modification method that is from theperspective of a base station (e.g. SeNB) which provides service to auser equipment (UE) connected to multiple base stations.

DETAILED DESCRIPTION OF DISCLOSED EMBODIMENTS

Reference will now be made in detail to the present exemplaryembodiments of the disclosure, examples of which are illustrated in theaccompanying drawings. Wherever possible, the same reference numbers areused in the drawings and the description to refer to the same or likeparts.

FIG. 5 illustrates the hardware components of a UE in terms offunctional block diagrams in accordance with one of the exemplaryembodiments of the disclosure. The exemplary UE would include notlimited to a processing circuit 501, an analog to digital (A/D)/digitalto analog (D/A) converting circuit 502, a transmitter 503, a receiver504, one or more antennas 505, and a non-transitory storage medium ormemory unit (not shown) coupled to the processing circuit 501. The UEwould receive radio frequency (RF) data through the one or more antennas505 which would be coupled to the receiver 504 via a matching networkand a filter. The receiver 504 would be coupled to the A/D D/A converter502 and would down convert the RF data directly or indirectly intobaseband data. The baseband data is converted by the A/D converter 502into digital data which would be transmitted to the processing circuit501. The UE would also transmit data by transmitting digital data to theD/A converter 502 to be converted into baseband data. The baseband datawould be up converted into RF data by the transmitter 503 whichtransmits RF data via the one or more antennas 505.

The processing circuit 501 would include one or more processing meanssuch as a microprocessor, microcontroller, and application specificintegrated circuit (ASIC). Contained within the processing circuit 501is a multiple connectivity configuration module 506 which would bedirectly involved or indirectly assist in the execution of the proposedconnection modification method including the first, second, . . . ,sixth exemplary embodiments. The storage medium may store programmingcodes, buffered and permanent data related to the operations of theproposed connection modification method.

The term “user equipment” (UE) in this disclosure could representvarious embodiments which for example could include but not limited to amobile station, an advanced mobile station (AMS), a server, a client, adesktop computer, a laptop computer, a network computer, a workstation,a personal digital assistant (PDA), a tablet personal computer (PC), ascanner, a telephone device, a pager, a camera, a television, ahand-held video game device, a musical device, a wireless sensor, and solike. In some applications, a UE may be a fixed computer deviceoperating in a mobile environment, such as a bus, train, an airplane, aboat, a car, and so forth.

FIG. 6 illustrates the hardware components of a base station in terms offunctional block diagrams in accordance with one of the exemplaryembodiments of the disclosure. A base station may include not limited toa processing circuit 601, an A/D D/A converter circuit 602, atransmitter 603, a receiver 604, one or more antennas 605, and anon-transitory storage medium (not shown). Contained within theprocessing circuit 601 is a multiple connectivity configuration module606 which would be directly involved or indirectly assist in theexecution of the proposed connection modification method including thefirst, second, . . . , sixth exemplary embodiments. The writtendescriptions of these components would be similar to the descriptions ofa UE in FIG. 5 and thus would not be repeated.

The phrase “base station” in this disclosure could be synonymous with“eNB”, and also these descriptions could cover obvious variants such asa master base station, secondary base station, slave base station, macrocell base station, micro cell base station, pico cell base station,femto cell base station, and so like. It would also be apparent to thoseskilled in the art that other types of base stations could be applicableto achieve similar network access purposes such as an advanced basestation (ABS), a base transceiver system (BTS), an access point (AP), ahome base station, a relay station, a scatter, a repeater, anintermediate node, an intermediary, and/or satellite-basedcommunications base stations.

In order to solve the aforementioned problem, the disclosure provides amethod and related communication devices for processing changes in datatransmission and reception in order to cope with a change of type of aradio bearer that was serving a UE so as to solve the aforementionedproblem. In order to adequately cover different scenarios, six exemplaryembodiments are provided to elucidate various concepts of thedisclosure.

FIG. 7 illustrates a first exemplary embodiment of the disclosure. Thefirst exemplary embodiment is directed to a UE side response to a changefrom a SCG bearer to either a MCG bearer or a split bearer. The scenarioof FIG. 7 could also be illustrated by FIG. 1. In step S701, a MeNBwould transmit to the UE a first RRC message that would include a firstconfiguration to establish a first MCG bearer used between the UE andthe MeNB to communicate with each other. In step S702, a SeNB wouldtransmit to the MeNB, over an inter-eNB interface, a configurationmessage that includes a second configuration to configure a first SCGbearer used between the SeNB and the UE to communicate with each other.The inter-eNB interface could be, for example, the X2 interface commonlyused by a LTE communication system. In step S703, the MeNB wouldtransmit to the UE a second RRC message that would include the secondconfiguration to configure the first SCG bearer.

After the first SCG bearer and the first MCG bearer have beenestablished by the UE to dually communicate with the MeNB and the SeNB,in step S704, the UE may encrypt, by using a first security key, a firstplurality of Packet Data Convergence Protocol (PDCP) service data unit(SDU) to generate a first encrypted PDCP SDU which would then betransmitted to the MeNB via the first MCG bearer. The encryption of thefirst PDCP SDU would be accomplished by using a first transmitter hyperframe number (TX_HFN) and a first PDCP sequence number (SN) associatedwith the first PDCP SDU. The UE sets the first PDCP SN according to afirst Next_PDCP_TX_SN before the encryption. The UE may encrypt a firstplurality of PDCP SDUs of the first MCG bearer to generate the firstplurality of encrypted PDCP SUDs which would then be transmitted to theMeNB, by using the first security key, the first TX_HFN and a PDCP SNassociated with each of the first plurality of PDCP SDUs. The UE setsthe PDCP SN associated with each of the first plurality PDCP SDUsaccording to the first Next_PDCP_TX_SN before encrypting each of thefirst plurality of PDCP SDUs. In step S705, the UE may encrypt, by usinga second security key, a second PDCP SDU by using a second TX_HFN and asecond PDCP SN associated with the second PDCP SDU to generate a secondencrypted PDCP SDU which would then be transmitted to the SeNB via thefirst SCG bearer. The UE sets the second PDCP SN according to a secondNext_PDCP_TX_SN. The second security key is used by the SeNB and the UEto communicate with each other. The UE may encrypt a second plurality ofPDCP SDUs of the first SCG bearer to generate the second plurality ofencrypted PDCP SUDs which would then be transmitted to the SeNB, byusing the second security key, the second TX_HFN and a PDCP SNassociated with each of the second plurality of PDCP SDUs. The UE setthe PDCP SN associated with each of the second plurality PDCP SDUsaccording to the second Next_PDCP_TX_SN.

Steps S706˜S709 would involve a change of the first SCG bearer into asecond MCG bearer or a split bearer. In step S706, the MeNB maydetermine to reconfigure the first SCG bearer to be a second MCG beareror a split bearer for the UE so the MeNB may transmit a message to theSeNB to release the first SCG bearer. In step S707, the MeNB transmitsto the UE a third RRC message which includes necessary information toreconfigure the first SCG bearer to be the second MCG bearer or thesplit bearer. The step of S707 may cause a third PDCP SDU, encrypted bythe second security key, the second TX_HFN, and a third PDCP SNassociated with the third PDCU SDU, to be unsuccessfully transmittedfrom the UE to the SeNB as shown in step S708 since the UE stopstransmitting any PDCP SDU of the first SCG bearer to the SeNB beforesuccessfully transmitting the encrypted third PDCP SDU uponreconfiguration of the first SCG bearer to be the second MCG bearer orthe split bearer. In step S709, after reconfiguring the first SCG bearerto be the second MCG bearer or the split bearer according to the thirdRRC message, the UE would then encrypt a fourth PDCP SDU of the secondMCG bearer or the split bearer by using the first security key as wellas the second TX_HFN and a fourth PDCP SN associated with the fourthPDCP SDU and subsequently transmit to the MeNB the encrypted fourth PDCPSDU. The UE sets the fourth PDCP SN according to the secondNext_PDCP_TX_SN. In other words, the UE keeps using the second TX_HFNand the second Next_PDCP_TX_SN of the first SCG bearer for the secondMCG bearer or the split bearer. Accordingly the MeNB sets its RX_HFN andNext_RX_PDCP_SN of the second MCG bearer or the split bearer to anRX_HFN and a Next_RX_PDCP_SN of the first SCG bearer respectively. TheMeNB may receive the RX_HFN and Next_RX_PDCP_SN of the first SCG bearerfrom the SeNB for receiving the encrypted fourth PDCP SDU. Alternativelythe UE uses a third TX_HFN and a third Next_TX_PDCP_SN which are set toinitial values (e.g. 0 s) by the UE for the second MCG bearer or thesplit bearer in response to step S707. In step 709, the UE would thenencrypt the fourth PDCP SDU by using the first security key as well asthe third TX_HFN and the fourth PDCP SN and subsequently transmit to theMeNB the encrypted fourth PDCP SDU. Before the encryption, the UE setsthe fourth PDCP SN according to the third Next_PDCP_TX_SN. Accordinglythe MeNB sets the RX_HFN and Next_RX_PDCP_SN of the second MCG bearer orthe split bearer to initial values (e.g. 0 s). It is noted the change ofthe first SCG bearer into a second MCG bearer or a split bearer may ormay not involve SeNB release.

Prior to or subsequent to step S708, the UE may attempt to re-transmitthe third PDCP SDU by encrypting, using the first security key, thesecond TX_HFN and the third PDCP SN associated with the third PDCP SDUand were used in communication with the SeNB, to generate a fifthencrypted PDCP SDU. In this way, the first exemplary embodiment hasprovided a signaling mechanism to handle the SCG bearer change as wellas a re-transmission mechanism to ensure that no data is lost.Alternatively the UE may attempt to re-transmit the third PDCP SDU byencrypting, using the first security key, the third TX_HFN and a fifthPDCP SN. The UE sets the fifth PDCP SN according to the thirdNext_PDCP_TX_SN before the encryption. It is noted that the UE may notattempt to re-transmit the third PDCP SDU since the UE does not supportre-transmission of the third PDCP SDU or it may be late for the MeNB toreceive the third PDCP SDU due to that the third PDCP SDU is real-timedata.

Possible parameters used by the aforementioned bearer configurations aredescribed by TS 36.300 which is incorporated by reference.

FIG. 8 illustrates a second exemplary embodiment of the disclosure. Thesecond exemplary embodiment is directed to a UE side in response to achange from either a MCG bearer or a split bearer to a SCG bearer. Thescenario of FIG. 8 could also be illustrated by FIG. 1. In step S801,the MeNB may transmit a first RRC message that contains necessaryinformation for the UE to establish a first MCG bearer. In step S802,the MeNB may transmit a second RRC message that contains necessaryinformation for the UE to establish a second MCG bearer or a splitbearer. In step S803, the SeNB may transmit a first configurationmessage that includes a first SCG configuration for communicationbetween the SeNB and the UE. In response to receiving the firstconfiguration message, in step S804 the MeNB may transmit a third RRCmessage that contains necessary information for the UE to establish thefirst SCG bearer.

After the first MCG bearer, the second MCG bearer or the split bearer,and the first SCG bearer have been established, in step S805 the UE mayencrypt a first PDCP SDU by using a first security key, a first TX_HFNand a first PDCP SN associated with the first PDCP SDU to generate anencrypted first PDCP SDU which would subsequently be transmitted to MeNBvia the first MCG bearer. The first security key is used forcommunication between the UE and the MeNB. The UE sets the first PDCP SNaccording to a first Next_PDCP_TX_SN. In step S806, the UE may encrypt asecond PDCP SDU of the second MCG bearer or the split bearer by usingthe first security key, a second TX_HFN and a second PDCP SN associatedwith the second PDCP SDU to generate an encrypted second PDCP SDU whichwould subsequently be transmitted to MeNB. The UE sets the second PDCPSN according to a second Next_PDCP_TX_SN before the encryption. In stepS807, the UE may encrypt a third PDCP SDU by using the second securitykey, a third TX_HFN and a third PDCP SN associated with the third PDCPSDU to generate an encrypted third PDCP SDU which would subsequently betransmitted to the SeNB via the first SCG bearer. The UE sets the thirdPDCP SN according to a third Next_PDCP_TX_SN before the encryption.

Steps S808˜S811 would involve a change of the second MCG bearer or splitbearer into a second SCG bearer. In step S808, the MeNB may determine toreconfigure the second MCG bearer or the split bearer to be the secondSCG bearer by transmitting a message to request addition of the secondSCG bearer for the UE. In step S809, the MeNB transmits to the UE afourth RRC message that contains necessary information for the UE toreconfigure the second MCG bearer or split bearer to be the second SCGbearer. In step S810, during the reconfiguration of the second MCGbearer or the split bearer, the UE may encrypt a fourth PDCP SDU of thesecond MCG bearer or split the bearer by using the first security key, asecond TX_HFN and a fourth PDCP SN associated with the fourth PDCP SDU,but the encrypted fourth PDCP SDU would not be successfully transmittedto the MeNB. In step S811, after reconfiguring the second MCG bearer orthe split bearer into the second SCG bearer, the UE may encrypt a fifthPDCP SDU by using the second security key, the second TX_HFN and a fifthPDCP SN associated with the fifth PDCP SDU to generate an encryptedfifth PDCP SDU. The UE sets the fifth PDCP SN according to the secondNext_PDCP_TX_SN before the encryption. In other words, the UE keepsusing the second TX_HFN and the second Next_PDCP_TX_SN of the second MCGbearer or the split bearer for the second SCG bearer. The UE would thentransmit the encrypted fifth PDCP SDU to the SeNB via the second SCGbearer. Accordingly the SeNB sets its Rx_HFN and Next_RX_PDCP_SN of thesecond SCG bearer to an Rx_HFN and a Next_RX_PDCP_SN of the second MCGbearer or the split bearer respectively. The SeNB may receive the Rx_HFNand Next_RX_PDCP_SN of the second MCG bearer or the split bearer fromthe MeNB. Alternatively the UE uses a fourth TX_HFN and a fourthNext_RX_PDCP_SN which are set to an initial value (e.g. 0) for thesecond SCG bearer. In step S811, after reconfiguring the second MCGbearer or the split bearer into the second SCG bearer, the UE mayencrypt the fifth PDCP SDU by using the second security key, the fourthTX_HFN and the fifth PDCP SN associated with the fifth PDCP SDU andsubsequently transmits the encrypted fifth PDCP SDU. The UE sets thefifth PDCP SN according to the fourth Next_PDCP_TX_SN before theencryption. Accordingly the SeNB sets the RX_HFN and Next_RX_PDCP_SN ofthe second SCG bearer to initial values (e.g. 0 s).

Prior to or subsequent to step S811, the UE may attempt to re-transmitthe fourth PDCP SDU which has been unsuccessfully transmitted previouslyby encrypting, using the second security key, the second TX_HFN and thefourth PDCP SN, which are associated with the fourth PDCP SDU and wereused in communication with the MeNB, to generate the encrypted fourthPDCP SDU. In this way, the second exemplary embodiment has provided asignaling mechanism to handle a MCG/split bearer change to a SCG beareras well as a re-transmission mechanism to ensure that no data would belost. Alternatively the UE may attempt to re-transmit the fourth PDCPSDU by encrypting, using the second security key, the fourth TX_HFN anda sixth PDCP SN. The UE sets the sixth PDCP SN according to the fourthNext_PDCP_TX_SN before the encryption. It is noted that the UE may notattempt to re-transmit the third PDCP SDU since the UE does not supportre-transmission of the fourth PDCP SDU or it may be late for the SeNB toreceive the fourth PDCP SDU due to that the fourth PDCP SDU is real-timedata.

FIG. 9A illustrates a scenario of altering a SCG bearer as a result of aSeNB change in accordance with a third exemplary embodiment of thedisclosure. The exemplary communication system of FIG. 9A would includeat least but not limited to a MeNB 951, a S-SeNB 952, a T-SeNB 953, anda UE 954 which would attempt to simultaneously connect to the MeNB 951and one of the S-SeNB 952 and the T-SeNB 953. The third exemplaryembodiment would involve the UE 954 changing its connection from theS-SeNB 952 to the T-SeNB 953 while keeping its connection to the MeNB951.

FIG. 9B illustrates the signaling diagram of third exemplary embodimentwhich is directed to a change from a first SCG bearer to a second SCGbearer in response to a change of serving SeNB. In step S901, the MeNBmay transmit to the S-SeNB a first SCG addition request message torequest addition of the SeNB for establishing a first SCG bearer for theUE. The first SCG addition request message would request the S-SeNB toallocate radio resource of a cell of the S-SeNB to communicate with theUE. In response to receiving the first SCG addition request message, instep S902, the S-SeNB would generate a first SCG configuration forconfiguring the first SCG bearer to the UE. The S-SeNB would thentransmit to the MeNB a first SCG addition response message whichincludes the first SCG configuration for establishing the first SCGbearer. In response to receiving the first SCG addition responsemessage, in S903 the MeNB would transmit to the UE a first RRC messagewhich includes the first SCG configuration for the UE to establish thefirst SCG bearer.

After the SCG bearer has been established, steps S904˜S907 involve theSCG change from the S-SeNB to T-SeNB, i.e. a SeNB change. In step S904,the MeNB may have determined to perform the SeNB change and subsequentlyto initiate the SeNB change by transmitting to the T-SeNB a second SCGaddition request message which would request addition of the T-SeNB as aserving SeNB for the UE. In order to provide the first SCG configurationfor the T-SeNB to establish the second SCG bearer, the MeNB would needto store the first SCG configuration when the first SCG configurationwas received from the S-SeNB. In step S904, the MeNB would transmit tothe T-SeNB the second SCG addition request message that includes thefirst SCG configuration. In step S905, in response to receiving thesecond SCG addition request message, the T-SeNB would transmit to theMeNB a second SCG addition response message which includes a second SCGconfiguration that is based on the first SCG configuration. In stepS906, the MeNB subsequently transmits to the S-SeNB a first SCG releaserequest message which would effectively release the S-SeNB. In responseto receiving the second SCG addition response message, in step S907, theMeNB would transmit to the UE a second RRC message that includes thesecond SCG configuration in order to establish the second SCG bearer.

It should be noted that the second SCG configuration could be generatedbased on the first SCG configuration by having the second SCGconfiguration to include only information that is different from thefirst SCG configuration. For example, if the first SCG configurationincludes {configuration 1, configuration 2, and configuration 3}, thesecond SCG configuration would include {configuration 2′} since theT-SeNB may attempt to apply the same configuration 1 and configuration 3as the S-SeNB but change configuration 2 to configuration 2′. Forexample, the configuration 1 may be a DRB identity and the configuration3 may be an EPS bearer identity. The configuration 2 may be aconfiguration related to Medium Access Control (MAC). Descriptions ofthe aforementioned configurations as well as possible parameters of thefirst SCG configuration and the second SCG configuration are describedin TS 36.300 and TS 36.331 which are incorporated by reference.

FIG. 10 illustrates a fourth exemplary embodiment which is an extensionof the scenario of altering a SCG bearer as a result of a SeNB changeand is similar to the third exemplary embodiment of the disclosure. Instep S1001, an S-SeNB would transmit a first SCG configuration messageto a MeNB. In step S1002, the MeNB would transmit to a UE a first RRCmessage which includes the first SCG configuration. The UE wouldestablish the first SCG bearer according to the first SCG configuration,steps S1003˜S1011 would involve a SCG change from the S-SeNB to aT-SeNB, i.e. a SeNB change.

In step S1003, the MeNB may determine the SeNB change and thus transmitto the T-SeNB a SCG addition request message which requests addition ofthe T-SeNB as a serving SeNB for the UE. In response to receiving theSCG addition request message, in step S1004, the T-SeNB would transmit aSCG addition response message which includes a second SCG configurationfor establishing a second SCG bearer between the T-SeNB and the UE. Inresponse to the event of S1004, the MeNB transmits to the S-SeNB a SCGrelease request which requests release of S-SeNB for the UE as stepS1005. In step S1006, the S-SeNB would encrypt a first PDCP SDU into afirst encrypted PDCP SDU and transmit the first encrypted PDCP SDU ofthe first SCG bearer, to the UE but the release of the first SCG bearerback in step S1005 would cause the first PDCP SDU to be not received bythe UE. In step S1007, the S-SeNB would transmit the first PDCP SDU tothe MeNB. In response to the event of S1007, in step S1008 the MeNBwould route the first PDCP SDU to the T-SeNB, and also in step S1009 theMeNB would transmit a second RRC message which includes the second SCGconfiguration in order to reconfigure the first SCG bearer to be thesecond SCG bearer for communication between the T-SeNB and the UE. Instep S1010, the T-SeNB would encrypt the first PDCP SDU into a secondencrypted PDCP SDU and transmit to the UE the second encrypted PDCP SDUso that the first PDCP SDU which is not successfully received by the UEis retransmitted to the UE by the T-SeNB. It is noted that the S-SeNBand the T-SeNB use different security keys to encrypt the first PDCPSDU. In step S1011, the T-SeNB would encrypt a second PDCP SDU of thesecond SCG bearer into a third encrypted PDCP SDU and transmit the thirdencrypted PDCP SDU to the UE. It is noted that the S-SeNB may notperform step S1007 for retransmission of the first PDCP SDU in the MeNBfor many reasons. For example, it may be late for the UE to receive thefirst PDCP SDU from the T-SeNB since the first PDCP SDU is real-timedata or the S-SeNB does not support forwarding of the first PDCP SDU tothe MeNB.

Moreover, after the T-SeNB establishes the second SCG bearer for the UE,the T-SeNB would encrypt a PDCP SDU of the second SCG bearer into anencrypted PDCP SDU transmitted using a security key, a TX_HFN and aNext_PDCP_TX_SN. The T-SeNB may initialize the TX_HFN andNext_PDCP_TX_SN by setting the TX_HFN and Next_PDCP_TX_SN to TX_HFN andNext_PDCP_TX_SN of the first SCG bearer respectively before using thesecond SCG bearer. Thus, in response to the determination in step S1005,the S-SeNB may transmit to the MeNB information indicating the TX_HFNand Next_PDCP_TX_SN of the first SCG bearer associated to a PDCP entityof the first SCG bearer. The MeNB would forward the information to theT-SeNB. In this case the UE keeps using a RX_HFN and a Next_PDCP_RX_SNof the first SCG bearer for decrypting the encrypted PDCP SDU receivedfrom the T-SeNB after step S1009. Alternatively the T-SeNB may set theTX_HFN and the Next_PDCP_TX_SN to initial values (e.g. zeros) for thesecond SCG bearer in response to step S1003. In this case, the UE alsoinitializes the RX_HFN and the Next_PDCP_RX_SN to the initial values(e.g. 0 s) for the second SCG bearer in response to step S1009.

Assuming that the first PDCP SDU were a first group of PDCP SDUs, thePDCP SNs associated with the first group of PDCP SDUs can be eachindividual PDCP SNs or could be derived from a starting PDCP SN of thePDCP SNs. In other words, the starting PDCP SN could be the lowest PDCPSN of a PDCP SDU which has not been transmitted successfully. The PDCPSNs associated with the first group of PDCP SDUs could also be derivedfrom a bitmap or other forms. According to the third as well as thefourth exemplary embodiments in general, a T-SeNB may transmit orre-transmit a plurality of PDCP SDUs to the UE in order to avoid lossesof PDCP SDUs as a result of the SCG change.

The fourth exemplary embodiment may further include the S-SeNBtransmitting to the MeNB RX_HFN(s) and PDCP SN(s) that are associatedwith PDCP SDU(s) which is associated with the first SCG bearer but notreceived by the MeNB in response to the SCG release request message. TheMeNB in response to receiving the RX_HFN(s) and the PDCP SN(s) maytransmit them to the T-SeNB. The T-SeNB may transmit information to theUE to indicate that the PDCP SDU(s) have not been received according tothe PDCP SN(s) of the PDCP SDU(s) if needed. Subsequently the UE may(re-)transmit the PDCP SDU(s) in order to avoid losses of the PDCPSDU(s) as a result of the SCG change by encrypting the PDCP SDU(s) intoencrypted PDCP SDU(s) using a security key known by the T-SeNB and(re-)transmitting the encrypted PDCP SDU(s) to the S-SeNB. The securitykey known by the T-SeNB is different from another security key was usedfor communication between the UE and the S-SeNB.

FIG. 11 illustrates a fifth exemplary embodiment which is directed to ascenario involving a change from a SCG bearer into a MCG bearer a splitbearer among multiple existing radio bearers. The scenario of the fifthexemplary embodiment could be represented by FIG. 1. In step S1101, theMeNB would transmit to the UE a first RRC message which would include afirst MCG configuration for establishing a first MCG bearer. A firstsecurity key is used for encrypting and decrypting data forcommunications between the MeNB and the UE. In step S1102, the MeNBwould receive from the SeNB a configuration message which includes afirst SCG configuration, a second SCG configuration. The first SCGconfiguration and the second SCG configuration are for establishing thefirst SCG bearer and the second SCG bearer respectively between the UEand the SeNB. A second security key is used for encrypting anddecrypting data for communications between the SeNB and the UE. In stepS1103, in response to receiving the configuration message from the SeNB,the MeNB would transmit to the UE a second RRC message which wouldinclude the first SCG configuration and the second SCG configuration.Assuming that the first SCG bearer, the second SCG bearer, and the firstMCG bearer have been established, step S1104˜1108 involve a change to aSCG bearer.

In step S1104, the MeNB determines to reconfigure the first SCG bearerto be a second MCG bearer or a split bearer and would transmit to theSeNB a SCG modification request message which would release the firstSCG bearer. In step S1105, the SeNB would encrypt a first PDCP SDU intoa first encrypted PDCP SDU by the second key and transmit to the UE thefirst encrypted PDCP SDU via the first SCG bearer; however, thedetermination back in step S1104 would cause the first encrypted PDCPSDU to be not successfully received by the UE. In response to stepS1104, in step S1106 the MeNB may transmit to the UE a third RRC messagethat includes necessary information to reconfigure the first SCG bearerto be the second MCG bearer or the split bearer. In step S1107, the SeNBwould transmit the first PDCP SDU to the MeNB. In response to receivingthe first PDCP SDU, in step S1108, the MeNB would encrypt, by using thefirst security key, the first PDCP SDU into a second encrypted PDCP SDUand transmit the encrypted second PDCP SDU to the UE. The first securitykey is used for communication between the MeNB and the UE. In stepS1109, the MeNB would encrypt a third PDCP SDU of the first MCG bearerinto a third encrypted PDCP SDU and a fourth PDCP SDU of the second MCGbearer or the split bearer into a fourth encrypted PDCP SDU by the firstsecurity key, and transmit to the UE the third and the fourth PDCP SDUsvia the first MCG bearer and the second MCG bearer respectively. It isnoted that the SeNB may not perform step S1107 for retransmission of thefirst PDCP SDU in the MeNB for many reasons. For example, it may be latefor the UE to receive the first PDCP SDU from the MeNB since the firstPDCP SDU is real-time data or the SeNB does not support forwarding ofthe first PDCP SDU to the MeNB.

Moreover, after the MeNB configures the second MCG bearer or the splitbearer for the UE, the MeNB would encrypt a PDCP SDU into an encryptedPDCP SDU transmitted via the second MCG bearer or the split bearer usingthe first security key, a TX_HFN and a Next_PDCP_TX_SN. The MeNB mayinitialize the TX_HFN and Next_PDCP_TX_SN to TX_HFN by setting theTX_HFN and Next_PDCP_TX_SN to TX_HFN and Next_PDCP_TX_SN of the firstSCG bearer respectively before the encryption. Thus, in response to thedetermination in step S1104, the SeNB may transmit to the MeNBinformation indicating the TX_HFN and Next_PDCP_TX_SN of the first SCGbearer. The MeNB would set the TX_HFN and Next_PDCP_TX_SN according tothe information. In this case the UE sets a RX_HFN and a Next_PDCP_RX_SNof the second MCG bearer or the split bearer to a RX_HFN and aNext_PDCP_RX_SN of the first SCG bearer respectively for decrypting theencrypted PDCP SDU received via the second MCG bearer or the splitbearer after step S1106. Alternatively the MeNB may initialize TX_HFNand the Next_PDCP_TX_SN to initial values (e.g. zeros) for the secondMCG bearer or the split bearer in response to step S1106. In this case,the UE also initializes the RX_HFN and Next_PDCP_RX_SN to the initialvalues (e.g. zeros) for the second MCG bearer or the split bearer inresponse to step S1106.

Assuming that the first PDCP SDU were a first group of PDCP SDUs, forsome of the first group of PDCP SDUs were not successfully deliveredback in step S1105, the PDCP SNs associated with the first group of PDCPSDUs can be each individual PDCP SNs or could be derived from a startingPDCP SN of the PDCP SNs. In other words, the starting PDCP SN could bethe lowest PDCP SN of a PDCP SDU which has not been transmittedsuccessfully. The PDCP SNs associated with the first group of PDCP SDUscould also be derived from a bitmap or other forms. Alternatively, theMeNB may set the PDCP SNs associated with the first group of PDCP SDUsaccording to the Next_PDCP_TX_SN which is initialized to an initialvalue (e.g. zero). In this case, the SeNB does not need to provideinformation of the PDCP SNs. The MeNB may transmit or re-transmit thefirst group of PDCP SDUs to the UE in order to avoid losses of PDCP SDUsas a result of a SCG bearer change to a MCG bearer or split bearer.

The fifth exemplary embodiment may further include the SeNB transmittingto the MeNB RX_HFN(s) and PDCP SN(s) that are associated with PDCPSDU(s) which is associated with the first SCG bearer but not received bythe SeNB in response to the SCG modification request message of stepS1104. The MeNB may transmit information to the UE to indicate that thePDCP SDUs have not been received according to the PDCP SNs of the PDCPSDUs. Subsequently the UE may (re-)transmit the PDCP SDUs in order toavoid losses of the PDCP SDUs as a result of a change of bearer type byencrypting the PDCP SDU(s), using the first security key, into encryptedPDCP SDU(s).

FIG. 12 illustrates a sixth exemplary embodiment which is directed to ascenario involving a change from either a MCG bearer or a split bearerinto a SCG bearer among multiple existing radio bearers. The scenario ofthe sixth exemplary embodiment could be represented by FIG. 1. In stepS1201, the MeNB would transmit a first RRC message or first two RRCmessages of which each includes necessary information to establish afirst MCG bearer and either a second MCG bearer or a split bearer. Afirst security key is used by the UE and the SeNB respectively forencrypting and decrypting data for communications between the UE and theMeNB. In step S1202, the SeNB would transmit a configuration messagethat includes a first SCG configuration. A second security key is usedby the UE and the SeNB respectively for encrypting and decrypting datafor communications between the UE and the SeNB. In step S1203, inresponse to receiving the configuration message, the MeNB would transmitto the UE a second RRC message which includes necessary information toestablish a first SCG bearer. After the first SCG bearer, the first MCGbearer, and the split bearer have been established, steps S1204˜S1208involve a change of a MCG bearer or a split bearer into a SCG bearer.

In step S1204A, the MeNB determines to reconfigure the second MCG beareror the split bearer to be a second SCG bearer and would transmit to theSeNB a SCG modification request message which would establish a secondSCG bearer. In step S1204B, the MeNB would receive an acknowledgment ofthe step S1204A by receiving a SCG modification response message fromthe SeNB. In step S1205, the MeNB would transmit to the UE a first PDCPSDU of the first MCG bearer or the split bearer which was encrypted byusing the first security key, but the determination back in step S1204Ato reconfigure the second MCG bearer or the split bearer to be thesecond SCG bearer would cause the first PDCP SDU not to be receivedsuccessfully by the UE. In response to step S1205, the MeNB would instep S1206 transmit to the SeNB the first PDCP SDU. In step S1207, theMeNB would transmit to the UE a third RRC message that includesnecessary information to reconfigure the second MCG bearer or the splitbearer to be the second SCG bearer. In step S1208, the SeNB wouldencrypt the first PDCP SDU by using the second security key to generatea second encrypted PDCP SDU and would subsequently transmit the secondencrypted PDCP SDU to the UE via the second SCG bearer. In step S1209,the SeNB may encrypt a third PDCP SDU of the first SCG bearer by usingthe second security key and subsequently transmit to the UE the thirdencrypted PDCP SDU, and encrypt the a fourth PDCP SDU of the second SCGbearer by using the second security key and subsequently transmit to theUE the fourth encrypted PDCP SDU. In step S1210, the MeNB may encrypt afifth PDCP SDU of the first MCG bearer by using the first security keyand subsequently transmit to the UE the fourth PDCP SDU. It is notedthat the MeNB may not perform step S1206 for retransmission of the firstPDCP SDU in the SeNB for many reasons. For example, it may be late forthe UE to receive the first PDCP SDU from the SeNB since the first PDCPSDU is real-time data or the MeNB does not support forwarding of thefirst PDCP SDU to the SeNB.

Moreover, after the SeNB establishes the second SCG bearer for the UE,the SeNB would encrypt a PDCP SDU transmitted via the second SCG bearerusing the second security key, a TX_HFN and a Next_PDCP_TX_SN. The SeNBmay initialize the TX_HFN and Next_PDCP_TX_SN by setting the TX_HFN andNext_PDCP_TX_SN to TX_HFN and Next_PDCP_TX_SN of the second MCG beareror the split bearer respectively before the encryption. Thus, the MeNBmay indicate to the SeNB that the TX_HFN and Next_PDCP_TX_SN of thesecond MCG bearer or the split bearer either in step S1204A or inanother message. The SeNB would set the TX_HFN and Next_PDCP_TX_SNaccording to the information for the second SCG bearer. In this case theUE sets a RX_HFN and a Next_PDCP_RX_SN of the second SCG bearer to aRX_HFN and a Next_PDCP_RX_SN of the second MCG bearer or the splitbearer respectively for decrypting the encrypted PDCP SDU received viathe second SCG bearer after step S1207. Alternatively the SeNB mayinitialize the TX_HFN and the Next_PDCP_TX_SN by setting the TX_HFN andthe Next_PDCP_TX_SN to initial values (e.g. zeros) for the second SCGbearer in response to step S1207. In this case, the UE also initializesRX_HFN and the Next_PDCP_RX_SN to the initial values (e.g. zeros) forthe second SCG bearer in response to step S1207.

Assuming that the first PDCP SDU were a first group of PDCP SDUs, forsome of the first group of PDCP SDUs were not successfully deliveredback in step S1205, the PDCP SNs associated with the first group of PDCPSDUs can be each individual PDCP SNs or could be derived from a startingPDCP SN of the PDCP SNs. In other words, the starting PDCP SN could bethe lowest PDCP SN of a PDCP SDU which has not been transmittedsuccessfully. The PDCP SNs associated with the first group of PDCP SDUscould also be derived from a bitmap or other forms received from theMeNB. According to the sixth exemplary embodiments in general, a SeNBmay transmit or re-transmit a plurality of PDCP SDUs to the UE in orderto avoid losses of PDCP SDUs as a result of a MCG bearer or a splitbearer change to a SCG bearer. Alternatively, the SeNB may set the PDCPSNs associated with the first group of PDCP SDUs according to theNext_PDCP_TX_SN which is initialized to an initial value (e.g. zero). Inthis case, the MeNB does not need to provide information of the PDCPSNs.

The sixth exemplary embodiment may further include the MeNB transmittingto the SeNB RX_HFN(s) and PDCP SN(s) that are associated with PDCPSDU(s) which is associated with the second MCG bearer or the splitbearer but not received by the MeNB in response to the SCG modificationrequest message of step S1204A. The SeNB may transmit information to theUE to indicate that the PDCP SDUs have not been received according tothe PDCP SNs of the PDCP SDUs if needed. Subsequently the UE may(re-)transmit the PDCP SDUs in order to avoid losses of the PDCP SDUs asa result of a change of bearer type by encrypting the PDCP SDU(s), usingthe second security key, into encrypted PDCP SDU(s).

FIG. 13 illustrates a connection modification method from theperspective of a user equipment (UE) capable of dually connecting to afirst and a second base stations. The method is consistent with thefirst exemplary embodiment as aforementioned and would include S1301receiving a first bearer configuration configuring a first master cellgroup (MCG) bearer for communication with the first base station fromthe first base station; S1302 configuring a first radio bearer as thefirst MCG bearer according to the first bearer configuration; S1303receiving a second bearer configuration configuring a secondary cellgroup (SCG) bearer for communication with the second base station fromthe first base station; S1304 configuring a second radio bearer as theSCG bearer according to the second bearer configuration; S1305generating a first encrypted Packet Data Convergence Protocol (PDCP)service data unit (SDU) by encrypting a first PDCP SDU of the firstradio bearer by using a first security key, a first transmitter hyperframe number (TX_HFN) and a first PDCN sequence number (SN) associatedwith the first PDCP SDU; S1306 transmitting the first encrypted PDCP SDUto the first base station; S1307 generating a second encrypted PDCP SDUby encrypting a second PDCP SDU of the second radio bearer by using asecond security key, a second TX_HFN and a second PDCP SN associatedwith the second PDCP SDU; S1308 transmitting the second encrypted PDCPSDU to the second base station; S1309 receiving a radio resource control(RRC) message comprising a third bearer configuration re-configuring theSCG bearer to be either a second MCG bearer or a split bearer from thefirst base station; S1310 re-configuring the second radio bearer eitheras the second MCG bearer or the split bearer according to the thirdbearer configuration; S1311 generating a third encrypted PDCP SDU byencrypting a third PDCP SDU of the second radio bearer by using thefirst security key, the second TX_HFN and a third PDCP SN associatedwith the third PDCP SDU, after the second radio bearer is reconfiguredto the second MCG bearer or the split bearer; and S1312 transmitting thethird encrypted PDCP SDU to the first base station.

Moreover, the third PDCP SDU would be encrypted using the secondsecurity key, the second TX_HFN, and the third PDCP SN to be transmittedto the second base station in response to or only in response to beingunsuccessfully transmitted to the second base station before the secondradio bear is reconfigured to be the second SCG bearer. Also, the thirdPDCP SN would be set according to a Next_PDCP_TX_SN associated with theSCG bearer.

FIG. 14 illustrates a connection modification method from theperspective of a user equipment (UE) capable of dually connecting to afirst and a second base stations. The method is consistent with thesecond exemplary embodiment as aforementioned and would include S1401receiving a first bearer configuration configuring a first master cellgroup (MCG) bearer for communication with the first base station fromthe first base station; S1402 configuring a first radio bearer as thefirst MCG bearer according to the first bearer configuration; S1403receiving a second bearer configuration configuring a second MCG beareror a split bearer for communication with the first base station from thefirst base station; S1404 configuring a second radio bearer as thesecond MCG bearer or the split bearer according to the second bearerconfiguration; S1405 receiving a third bearer configuration configuringa first secondary cell group (SCG) bearer for communication with thesecond base station, from the first base station; S1406 configuring athird radio bearer as the first SCG bearer according to the third bearerconfiguration; S1407 generating a first encrypted Packet DataConvergence Protocol (PDCP) service data unit (SDU) by encrypting afirst PDCP SDU of the first radio bearer by using a first security key,a first transmitter hyper frame number (TX_HFN) and a first PDCPsequence number (SN) associated with the first PDCP SDU; S1408transmitting the first encrypted PDCP SDU to the first base station;S1409 generating a second encrypted PDCP SDU by encrypting a second PDCPSDU of the second radio bearer by using the first security key, a secondTX_HFN and a second PDCP SN associated with the second PDCP SDU; S1410transmitting the second encrypted PDCP SDU to the first base station;S1411 generating a third encrypted PDCP SDU by encrypting a third PDCPSDU of the third radio bearer by using a second security key, a thirdTX_HFN and a third PDCP SN associated with the third PDCP SDU; S1412transmitting the third encrypted PDCP SDU to the second base station;S1413 receiving a radio resource control (RRC) message comprising afourth bearer configuration re-configuring the second MCG bearer or thesplit bearer to be a second SCG bearer; S1414 re-configuring the secondradio bearer as a second SCG bearer according to the fourth bearerconfiguration; S1415 generating a fourth encrypted PDCP SDU byencrypting a fourth PDCP SDU of the second radio bearer by using thesecond security key, the second TX_HFN and a fourth PDCP SN associatedwith the fourth PDCP SDU, after the second radio bearer is reconfiguredto the second SCG bearer; and S1416 transmitting the fourth encryptedPDCP SDU to the second base station.

Moreover the fourth PDCP SDU would be encrypted using the first securitykey, the second TX_HFN, and the fourth PDCP SN in response to or only inresponse to being unsuccessfully transmitted to the first base stationbefore the second radio bearer is reconfigured as the second SCG bearer.Also the fourth PDCP SN would be set according to a Next_PDCP_TX_SNassociated with the second MCG bearer or the split bearer.

FIG. 15 illustrates a connection modification method that is from theperspective of a first base station which serves a user equipment (UE)capable of dually connecting to the first base station and a second basestation. The method is consistent with the third and fourth exemplaryembodiments as aforementioned as would include S1501 receiving a firstSCG configuration for a UE from a second base station; S1502transmitting a first radio resource control (RRC) message comprising thefirst SCG configuration to establish a first SCG bearer, to the UE;S1503 transmitting a first SCG addition request comprising the first SCGconfiguration for establishing a second SCG bearer for the UE, to athird base station; S1504 receiving a first SCG addition responsecomprising a second SCG configuration which is based on the first SCGconfiguration in response to transmitting the first SCG additionrequest, from the third base station; S1505 transmitting a SCG releaserequest to release the first SCG bearer, to the second base station; andS1506 transmitting a second RRC message comprising the second SCGconfiguration to establish the second SCG bearer, to the UE.

Moreover, receiving the first SCG configuration would includetransmitting a second SCG addition request to the second base stationfor establishing the first SCG bearer with the UE and receiving a secondSCG addition response comprising the first SCG configuration from thesecond base station. The second SCG configuration may includeinformation that is different from the first SCG configuration. Also,after transmitting the SCG release request to release the first SCGbearer, the first base station may further receive, from the second basestation, a PDCP SDU of the first SCG bearer, a TX_HFN and a PDCP SN, inresponse to or only in response to the PDCP SDU being unsuccessfullytransmitted previously. The TX_HFN and PDCP SN would be associated withthe PDCP SDU, and the first base station may also transmit to the thirdbase station the PDCP SDU of the first SCG bearer, the TX_HFN and thePDCP SN in response to receiving from the second base station the PDCPSDU of the first SCG bearer, the TX_HFN and the PDCP SN.

Moreover, the first base station may receive from the second basestation, a receiver hyper frame number (RX_HFN) and a PDCP SN which areassociated with another PDCP SDU which is not received by the secondbase station in response to or only in response to transmitting the SCGrelease request and would also transmit to the third base station, theRX_HFN and the PDCP SN which are associated with the other PDCP SDU.

FIG. 16 illustrates a connection modification method that is from theperspective of a base station of multiple base stations which providesservice to a user equipment (UE) connected to the multiple basestations. The method is consistent with the fifth exemplary embodimentsas aforementioned as would include S1601 transmitting a first radioresource control (RRC) message to configure a first master cell group(MCG) bearer for communication to a UE; S1602 transmitting to the UE afirst Packet Data Convergence Protocol (PDCP) service data unit (SDU)encrypted with a first security key; S1603 receiving a first secondarycell group (SCG) configuration configuring a first SCG bearer and asecond SCG configuration configuring a second SCG bearer, from anotherbase station; S1604 transmitting at least one second RRC message toconfigure the first SCG bearer and the second SCG bear in response toreceiving the first SCG configuration and the second SCG configuration;S1605 transmitting a message to release the first SCG bearer, to theother base station; S1606 receiving a transmitter hyper frame number(TX_HFN) and a Next_PDCP_TX_SN of the first SCG bearer, from the otherbase station; S1607 transmitting a third RRC message reconfiguring thefirst SCG bearer to be a second MCG bearer or a split bearer, to the UE;and S1608 transmitting to the UE a second PDCP SDU encrypted by usingthe first security key, the TX_HFN and the Next_PDCP_TX_SN, via the MCGbearer or the split bearer.

Moreover, the base station may further receive a receiver hyper framenumber (RX_HFN) and a PDCP sequence number (SN) associated with a thirdPDCP SDU which is associated with the first SCG bearer and not receivedby the other base station, from the other base station in response totransmitting the message.

FIG. 17 illustrates a connection modification method that is from theperspective of a base station (e.g. SeNB) of multiple base stationswhich provides service to a user equipment (UE) connected to themultiple base stations. The method is consistent with the sixthexemplary embodiments as aforementioned as would include S1701transmitting a first secondary cell group (SCG) configurationconfiguring a first SCG bearer for a UE, to another base station; S1702receiving a message requesting establishing a second SCG bearerreplacing a second MCG bearer or a split bearer of the UE from the otherbase station; S1703 receiving a transmitter hyper frame number (TX_HFN)and a Next_PDCP_TX_SN of the second MCG bearer or the split bearer, fromthe other base station; S1704 generating an encrypted PDCP SDU byencrypting a PDCP SDU using a security key, the TX_HFN and theNext_PDCP_TX_SN; and S1705 transmitting the encrypted PDCP SDU via thesecond SCG bearer to the UE.

Moreover, the base station may further receive a receiver hyper framenumber (RX_HFN) and a PDCP sequence number (SN) associated with the PDCPSDU which is associated with the second MCG bearer or the split bearerbut not received by the other base station in response to receiving themessage.

In view of the aforementioned descriptions, the present disclosure issuitable for being used in a wireless communication system and providesmechanisms to re-configure radio bearers, to encrypt/decrypt PDCP SDUsusing correct security key and to re-transmit PDCP SDUs so as to avoidlosses of PDCP SDUs as a result of a SCG change.

No element, act, or instruction used in the detailed description ofdisclosed embodiments of the present application should be construed asabsolutely critical or essential to the present disclosure unlessexplicitly described as such. Also, it would be apparent to an ordinaryperson skilled in the art that sequences of the steps in each of theexemplary embodiments of the present disclosure could be changed inorder to adapt to various designing needs without departing from thespirit of the disclosure. Moreover, as used herein, each of theindefinite articles “a” and “an” could include more than one item. Ifonly one item is intended, the terms “a single” or similar languageswould be used. Furthermore, the terms “any of” followed by a listing ofa plurality of items and/or a plurality of categories of items, as usedherein, are intended to include “any of”, “any combination of”, “anymultiple of”, and/or “any combination of” multiples of the items and/orthe categories of items, individually or in conjunction with other itemsand/or other categories of items. Further, as used herein, the term“set” is intended to include any number of items, including zero.Further, as used herein, the term “number” is intended to include anynumber, including zero.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the structure of thedisclosed embodiments without departing from the scope or spirit of thedisclosure. In view of the foregoing, it is intended that the disclosurecover modifications and variations of this disclosure provided they fallwithin the scope of the following claims and their equivalents.

Moreover, the claims should not be read as limited to the describedorder or elements unless stated to that effect. In addition, use of theterm “means” in any claim is intended to invoke 35 U.S.C. § 112, ¶6, andany claim without the word “means” is not so intended.

What is claimed is:
 1. A connection modification method applicable to auser equipment (UE) which is capable of dually connecting to a first anda second base stations, the method comprising: receiving a first bearerconfiguration configuring a first master cell group (MCG) bearer forcommunication with the first base station from the first base station;configuring a first radio bearer as the first MCG bearer according tothe first bearer configuration; receiving a second bearer configurationconfiguring a secondary cell group (SCG) bearer for communication withthe second base station from the first base station; configuring asecond radio bearer as the SCG bearer according to the second bearerconfiguration; generating a first encrypted Packet Data ConvergenceProtocol (PDCP) service data unit (SDU) by encrypting a first PDCP SDUof the first radio bearer by using a first security key, a firsttransmitter hyper frame number (TX_HFN) and a first PDCN sequence number(SN) associated with the first PDCP SDU; transmitting the firstencrypted PDCP SDU to the first base station; generating a secondencrypted PDCP SDU by encrypting a second PDCP SDU of the second radiobearer by using a second security key, a second TX_HFN and a second PDCPSN associated with the second PDCP SDU; transmitting the secondencrypted PDCP SDU to the second base station; receiving a radioresource control (RRC) message comprising a third bearer configurationre-configuring the SCG bearer to be either a second MCG bearer or asplit bearer from the first base station; re-configuring the secondradio bearer either as the second MCG bearer or the split beareraccording to the third bearer configuration; generating a thirdencrypted PDCP SDU by encrypting a third PDCP SDU of the second radiobearer by using the first security key, the second TX_HFN and a thirdPDCP SN associated with the third PDCP SDU, after the second radiobearer is reconfigured to the second MCG bearer or the split bearer; andtransmitting the third encrypted PDCP SDU to the first base station,wherein the third PDCP SDU was encrypted using the second security key,the second TX_HFN, and the third PDCP SN, and was unsuccessfullytransmitted to the second base station.
 2. The method of claim 1 furthercomprising: setting the third PDCP SN according to a Next_PDCP_TX_SNassociated with the SCG bearer, wherein the Next_PDCP_TX_SN indicates aPDCP SN of the next PDCP SDU to be transmitted.
 3. A connectionmodification method applicable to a user equipment (UE) capable ofdually connecting to a first and a second base stations, the methodcomprising: receiving a first bearer configuration configuring a firstmaster cell group (MCG) bearer for communication with the first basestation from the first base station; configuring a first radio bearer asthe first MCG bearer according to the first bearer configuration;receiving a second bearer configuration configuring a second MCG beareror a split bearer for communication with the first base station from thefirst base station; configuring a second radio bearer as the second MCGbearer or the split bearer according to the second bearer configuration;receiving a third bearer configuration configuring a first secondarycell group (SCG) bearer for communication with the second base station,from the first base station; configuring a third radio bearer as thefirst SCG bearer according to the third bearer configuration; generatinga first encrypted Packet Data Convergence Protocol (PDCP) service dataunit (SDU) by encrypting a first PDCP SDU of the first radio bearer byusing a first security key, a first transmitter hyper frame number(TX_HFN) and a first PDCP sequence number (SN) associated with the firstPDCP SDU; transmitting the first encrypted PDCP SDU to the first basestation; generating a second encrypted PDCP SDU by encrypting a secondPDCP SDU of the second radio bearer by using the first security key, asecond TX_HFN and a second PDCP SN associated with the second PDCP SDU;transmitting the second encrypted PDCP SDU to the first base station;generating a third encrypted PDCP SDU by encrypting a third PDCP SDU ofthe third radio bearer by using a second security key, a third TX_HFNand a third PDCP SN associated with the third PDCP SDU; transmitting thethird encrypted PDCP SDU to the second base station; receiving a radioresource control (RRC) message comprising a fourth bearer configurationre-configuring the second MCG bearer or the split bearer to be a secondSCG bearer; re-configuring the second radio bearer as the second SCGbearer according to the fourth bearer configuration; generating a fourthencrypted PDCP SDU by encrypting a fourth PDCP SDU of the second radiobearer by using the second security key, the second TX_HFN and a fourthPDCP SN associated with the fourth PDCP SDU, after the second radiobearer is reconfigured to the second SCG bearer; and transmitting thefourth encrypted PDCP SDU to the second base station, wherein the fourthPDCP SDU was encrypted using the first security key, the second TX_HFNand the fourth PDCP SN, and was unsuccessfully transmitted to the firstbase station before the second radio bearer is reconfigured to thesecond SCG bearer.
 4. The method of claim 3 further comprising settingthe fourth PDCP SN according to a Next_PDCP_TX_SN associated with thesecond MCG bearer or the split bearer, wherein the Next_PDCP_TX_SNindicates a PDCP SN of the next PDCP SDU to be transmitted.
 5. Aconnection modification method applicable to a base station, the methodcomprising: transmitting a first radio resource control (RRC) message toconfigure a first master cell group (MCG) bearer for communication to aUE; transmitting to the UE a first Packet Data Convergence Protocol(PDCP) service data unit (SDU) encrypted with a first security key;receiving a first secondary cell group (SCG) configuration configuring afirst SCG bearer, from a second base station; transmitting at least onesecond RRC message to configure the first SCG bearer in response toreceiving the first SCG configuration, wherein the second base stationtransmits a third PDCP SDU encrypted by a second security key to the UEvia the first SCG bearer; transmitting a message to release the firstSCG bear, to the second base station; receiving a transmitter hyperframe number (TX_HFN) and a Next_PDCP_TX_SN of the first SCG bearer,from the other base station, wherein the Next_PDCP_TX_SN indicates aPDCP SN of the next PDCP SDU to be transmitted; transmitting a third RRCmessage reconfiguring the first SCG bearer to be a second MCG bearer ora split bearer, to the UE; transmitting to the UE a second PDCP SDUencrypted by using the first security key, the TX_HFN and theNext_PDCP_TX_SN, via the MCG bearer or the split bearer; and receiving areceiver hyper frame number (RX_HFN) and a PDCP sequence number (SN)associated with a third PDCP SDU which is associated with the first SCGbearer and not received, from the other base station in response totransmitting the message.
 6. A connection modification method applicableto a second base station, the method comprising: transmitting a firstsecondary cell group (SCG) configuration configuring a first SCG bearerfor a UE to a first base station; receiving a message requestingestablishing a second SCG bearer replacing a second MCG bearer or asplit bearer of the UE, from the first base station, wherein the firstbase station transmits a first PDCP SDU of the first MCG bearer or thesplit bearer which is encrypted by using a first security key to the UE;receiving a transmitter hyper frame number (TX_HFN) and aNext_PDCP_TX_SN of the second MCG bearer or the split bearer from thefirst base station, wherein the Next_PDCP_TX_SN indicates a PDCP SN ofthe next PDCP SDU to be transmitted; generating an encrypted PDCP SDU byencrypting a PDCP SDU using a second security key, the TX_HFN and theNext_PDCP_TX_SN; transmitting the encrypted PDCP SDU via the second SCGbearer to the UE; and receiving a receiver hyper frame number (RX_HFN)and a PDCP sequence number (SN) associated with the PDCP SDU which isassociated with the second MCG bearer or the split bearer but notreceived in response to receiving the message.